I assume that by 'external display' you mean the monitor on your Mac device.
I can think of two theoretical ways.
The first is using the network commands. The iOS device would have one app with the controls, HUD, etc. and then Mac would have the play area app. The Mac would need to be the server in the network communications. And you would use network commands to send information back and forth. In theory this could all be done with AppGameKit code.
The second theoretical method would be using blue tooth communications. I know there is demo code for doing blue tooth pairing and sending commands between Mac and device (and recently discovered iPad Worms and had a blue tooth network game with three other people, but it wasn't perfect). That would require definitely using Tier 2 and somehow making the hooks in the AppGameKit portion to get the information from the blue tooth side.
Cheers,
Ancient Lady
AGK Community Tester and AppGameKit Master